Cellulose and cellulose derivatives including cellulose ester and cellulose ether are now attracting considerable attention because they are major biomass-based materials and also because they can be biodegraded in the environment. Cellulose acetate, which is a well-known commercially available cellulose ester, has been used for many years in producing cigarette filters and fiber materials for clothing. Other cellulose esters include cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate phthalate, which are used widely as material for plastics, filters and coatings.
As fiber material, cellulose has been used from old times in the form of spun fiber using short fiber of naturally grown cotton and hemp. Methods to produce filament material instead of short fiber include wet spinning, used for instance to dissolve cellulose such as rayon in a special solvent such as carbon disulfide, and dry spinning, used for instance to produce a cellulose derivative such as cellulose acetate which is then dissolved in an organic solvent such as methylene chloride or acetone, followed by spinning while evaporating the solvent, and furthermore, a method has been disclosed (see JP-51-70316 A 1) in which a cellulose acetate melt containing a large amount of water-soluble plasticizer such as polyethylene glycol is subjected to melt spinning to produce for hollow fibers to be used as filter membrane. The latter method, however, often suffers severance of yarns during the spinning process, and has to use a low draft ratio to permit melt spinning, making it impossible to produce fiber with a sufficiently small fineness for common clothing. The method generally can produce thick yarns such as for hollow filaments for filter membrane, but very low in strength, they are stiff, less flexible, and easily broken if a fabric is produced, so it will be extremely difficult to manufacture clothing and other common products that require both a small fineness and a high strength.
If a hollow fiber for filter is produced from cellulose acetate containing as large as 20% of plasticizer, small pores will grow in the fiber in a subsequent treatment with water or alkali. However, these large number of pores will further decrease the strength of the fiber, and tend to cause whitening due to abrasion and a decrease in fastness, which is another reason for inappropriateness as material for clothing which suffers continuous external forces during use.
Cellulose acetate material produced by dry spinning generally suffers a very large deformation of fibers as a result of evaporation of solvents from inside the fiber immediately after the spinning, leading to indefinite cross-sections. Thus, acetate fabrics are inferior to polyester and other melt-spun fabrics composed of uniform fiber with controlled cross-sections in that the former has uneven surface quality with irregular features.
It has also been disclosed (see JP-11-506175T) that the use of the melt blow method for spinning of cellulose ester permits the spinning of yarns with a small fineness. However, though fiber structures produced by melt blow are widely used as industrial nonwoven fabrics, their applications are essentially very limited because such fiber cannot serve for production of woven and knitted fabrics. Further, the melt blow method has essential difficulty in achieving a uniform fiber diameter, and the coefficient of variation (CV) in fineness, which represents the unevenness in fineness, is 30 to 40% in most cases, indicating that the thickness of single fibers varies largely.
Thus, fabrics composed of yarns tend to vary in cross-section and fineness of the fiber, and it is difficult to achieve a uniformity in glossiness resulting from reflection of light on the surface and a uniformity in color resulting from dyeing, leading to perceived unevenness.
It is known that thin yarns with a uniform fineness, such as those conventionally used in clothing, can be produced by melt spinning with a high productivity if using a composition prepared by kneading, at a specific mixing ratio, a cellulose mixed ester and a plasticizer that is compatible to said cellulose mixed ester (see JP-2004-182979 A1).
On the other hand, a cellulose mixed ester containing a plasticizer has a low glass transition point (Tg), and is so low in heat resistance for daily-use clothing that heating during ironing can cause fusion easily. Containing a plasticizer, moreover, the fiber is so low in strength that if clothing is produced from a fabric of such fiber, it will be low in strength and will be easily torn.
Concerning the quality of a fabric for clothing, it is important to meet the requirements for aesthetic appeal and texture as well as the basic physical properties such as strength and heat resistance during use.
Thus, fabrics with high heat resistance, good yarn properties and aesthetic appeal that can be used as material for general clothing cannot be produced easily by subjecting cellulose, a biomass-based material, to a melt spinning process that is free of environmentally harmful solvents.